BMW’s Exhaust Cleaner than Surrounding Air in Hydrogen 7
https://www.geek.com/mobile/bmws-exhaust-cleaner-than-surrounding-air-in-hydrogen-7-1371749/

Therefore, the best place to ride is behind the exhaust pipe on a
clean air vehicle.

Personally, I believe the best solution is to turn the bicycle into an
electrostatic precipitator. Simply design a bicycle frame with the
appropriate insulation, and put a few thousand volts between the front
rack and the rear rack. The particles pickup a charge on the front
rack, and are trapped by the opposite polarity charge on the rear
rack. It's similar to a home "air purifier" but without the charcoal
filter. Or, just mount one of these on your bicycle:
http://www.airocide.com
https://www.amazon.com/Airocide-Purifier-allergies-sensitive-individuals/dp/B00CKXVHNO/

New bike reduces pollution by cleaning air as you cycle
https://www.independent.co.uk/news/bike-pollution-cleaning-air-bicycle-beijing-china-dan-roosegaarde-a7829891.html

Just one little problem. It isn't healthy riding around on a bicycle
while breathing polluted air:
https://www.theguardian.com/cities/2017/feb/13/tipping-point-cities-exercise-more-harm-than-good

Better living through (cough) technology.
Best not to live in a metropolis and then all those problems go away. As
long as you avoid cycling behind a school bus.

Our local skool busses have been running on CNG (compressed natural
gas) since 2002:
http://www.santacruzsentinel.com/general-news/20180601/santa-cruz-metro-unveils-new-buses
http://www.tpgonlinedaily.com/fta-awards-1-2m-to-metro-for-new-buses/
The weird thing about these buses is that they used to put something
in the gas that makes it smell like lilac flowers (or xylyl bromide
poison gas). It was strong enough to cover up any other exhaust
stench. However, I just realized that I haven't smelled the flowers
for several years, so I guess they stopped doing that. Or, they may
have moved the tail pipe to dump the fumes INSIDE the bus:
"More exhaust inhaled by kids inside school buses than by others in
the area, says study"
https://www.berkeley.edu/news/media/releases/2005/04/04_schoolbus.shtml

I moved out of the big city in about 1973 and have been living happily
in the overgrown forests of the Santa Cruz mountains. Back then, the
dogs used to sleep in the middle of the main highway. Today, it's
either a race track or a parking lot. I'm told that anxiety,
excitement, or panic bad for my heart, so I avoid riding on the major
highways. Mostly, I ride on the bike paths that parallel the river.
That should work until someone decided to run buses on the bike paths.

New bike reduces pollution by cleaning air as you cycle
https://www.independent.co.uk/news/bike-pollution-cleaning-air-bicycle-beijing-china-dan-roosegaarde-a7829891.html

Just one little problem. It isn't healthy riding around on a bicycle
while breathing polluted air:
https://www.theguardian.com/cities/2017/feb/13/tipping-point-cities-exercise-more-harm-than-good

Better living through (cough) technology.
Best not to live in a metropolis and then all those problems go away. As
long as you avoid cycling behind a school bus.

Our local skool busses have been running on CNG (compressed natural
gas) since 2002:
http://www.santacruzsentinel.com/general-news/20180601/santa-cruz-metro-unveils-new-buses
http://www.tpgonlinedaily.com/fta-awards-1-2m-to-metro-for-new-buses/
The weird thing about these buses is that they used to put something
in the gas that makes it smell like lilac flowers (or xylyl bromide
poison gas). It was strong enough to cover up any other exhaust
stench. However, I just realized that I haven't smelled the flowers
for several years, so I guess they stopped doing that. Or, they may
have moved the tail pipe to dump the fumes INSIDE the bus:
"More exhaust inhaled by kids inside school buses than by others in
the area, says study"
https://www.berkeley.edu/news/media/releases/2005/04/04_schoolbus.shtml

It's disgusting. When I see one at a red traffic light I stop 100ft back
and wait until it is in 2nd gear.

I moved out of the big city in about 1973 and have been living happily
in the overgrown forests of the Santa Cruz mountains. Back then, the
dogs used to sleep in the middle of the main highway. Today, it's
either a race track or a parking lot. I'm told that anxiety,
excitement, or panic bad for my heart, so I avoid riding on the major
highways. Mostly, I ride on the bike paths that parallel the river.
That should work until someone decided to run buses on the bike paths.